Electrical apparatus



June 2, 1931.

o. E. MARVEL ELECTRICAL APPARATUS Filed March so, 1925 4 Sheets-Sheet l June 2, 1931. MARVEL 7 1,807,995

ELECTRICAL APPARATUS 7 Filed March 50, 1925 4 Sheets-Sheet 2 June 2, 1931.

v o. E. MARVEL.

ELECTRICAL APPARATUS Filed March 30,

Zulu

1925 4 Sheets-Sheet 3 "mull/I11 gwuzm lio c June 2, 1931. o. E. MARVEL ELECTRICAL APPARATUS Filed March 50, 1925 4 Sheets-Sheet 4 Patented June 2, 1931 time , ORIN E. IiIABVEL, OF DAYTON, OHIO, ASS-IGNOR, BY ASSIGNMENTS, TO GENERAL IIIO'IORS RADIO CORPORATION, OF DAYTON,OI-IIO, A CORPORATION OF OHIO ELEOTEZGAL Application filed March 30, 1925. Serial No. 19,370.

The present invention relates to means for adjusting or controlling a plurality of control devices concurrently or in unison, and is particularly adapted for the control of g tuning devices such as are employed in radio apparatus.

One of the principal objects of the inven tion is to provide a primary or master control for actuating two or more control devices concurrently and for actuating certain of the control devices independently of the others.

Another object is to control a plurality of coordinated control devices in unison or singly and to automatically bring the control devices into coordination with one another after they have been moved out of coordination.

Other objects of the invention will be apparent from the following description when taken in connection with the accompanying drawings, in which Fig. 1 is a plan view of a preferred em bodiment of the invention, parts being shown in section or broken away;

Fig. 2 is a partly diagrannnatic elevation taken on the line 22 of 1;

Figs, 3 and 4 are enlarged vertical sections on the lines 33 and Moi Fig. 1;

Figs. 5 and 6 are enlarged-views showing details of construction of the apparatus illust-rated in Figs. 1 to 4;

Fig. 7 is a view corresponding to Fig. 2 showing a second embodiment of the invention; 7 v

Figs. 8 and 9 are enlarged vertical sec tions corresponding to Figs. 3 and 4 taken on the lines 8- S and 99 ofFig. 7; i

Fig, 10 is a plan view of still another form of the invention;

, Fig. 11 is a section taken on the line 11'11 of Fig. 10; and I r Figs. 12 to 15 show views illustrative of a further adaptation of the invention, Fig. 12 representing in schematic diagram electrical circuits which may be controlled by the means illustrated in Figs. 13 to 15, Figs. 13 and 14 being sectional views along substantially horizontal lines indicated at 13-13 and 14 14 respectively of Fig. 15, and Fig. 15 being a vertical section taken on the. line 1515 of Figs. 13 and 14. Referring particularly to Figs. 1 to 6 of the drawings 20 designates the panel of a radio receiving apparatus having mounted thereon a plurality of electrical controldevices 21, 22, and 23, which control devices may he means for tuning a plurality of tuned circuits. Such tuning devices may be in the form of condensers or other tuning devices, but for the purpose of the present in vention such devices are illustrated as con densers each having fixed condenser plates 2% and movable condenser plates 25 which may be. of the usual or conventional form but which are herein illustrated as semi-circular in shape. The movable condenser plates are fixedly mounted upon suitable r0- tatable shafts 26, 27, and 28 which extend through the panel 20 and are provided with operating handles or dials 29', 30, and 31 respectively. Indicating arms 32, 33, and 3 1 may be mounted on the rotatable condenser shafts to indicate the relative positions of the fixed and movable condenser plates, and also to indicate (when they point in the same direction) that the movable plates of the several condensers are coordinated with one another.

It will be understood that in the best forms of radio receiving apparatus in which the control'devices 21, 22, 311(1'23 are employed for tuning associated tuned circuits, the construction and arrangement of the parts is preferably such that equal angular movements of the several tuning devices or condensers 21, 22, and 23 result in equal changes in the periods or wave lengths of the several tuned circuits-controlled thereby. In other words when the indicating arms or pointers 32, 33, and .3 1 point to zero then all the tuned circuits are tuned to the same frequency, and likewise when the indicating arms point to any other graduation on the panel as for example when they all, point to 90, then the several tuned circuits are again tunedto the same frequency. In such an apparatus it is desirable to move or turn the movable elements of the condensers in unison,

be termed a driven pulley 46 which consists of an insulating hub portion 47 pinned to the condenser shaft at 48 a preferably grooved rim portion 49 mounted upon the circumferential edge of the insulated hub 47. As best shown in Fig. 4 the rim 49 comprises a disk portion 50 having a circular aperture therein for receiving the hub 47.

The rim 49 is maintained in frictional engagement with the hub 47 by means of a number of ,/spring supporting members 51 disposed about the circumference of the hub 47 thec onnect-ion being such that rotational movement of the rim 49 is transmitted to the condenser shaft 27'to cause a corresponding movement thereof. Movement of the condenser shaft 27 and hence movement of the condenser plates mounted thereon may be effected independently of the other condensersby turning the secondary control dial 30, the resistance to movement of. the rim 49, by reason of its connection with another pulley, being such as to hold the rim 49 immovable while the shaft 27 is being turned.

As best shown in Fig.1, the pulley rim 42 of condenser 21 is connected with thepulley rim 49 of the condenser 22 by belt 44, and the pulley rim 43 is connected with the pulley rim 49 of condenser 23 through belt 45. By this connection it is possible through rotation of the primary or master control dial 29 to rotate the several pulleys in unison, the pulleys 49 being driven" from the double rimmed "driving pulley 40 which is carried upon the shaft of condenser 21. To maintain the proper driving connection between the belts 44 and 45 and the several pulleys, the belts are preferably fixed to the pulleys at certain points,- though it will be understood that this is not essential. Thus as shown in Fig. 2 the belt 44 is fixed to the pulley 40 at 55 and it is secured to the pulley 49 of condenser 22 at 56. Likewise the belt 45 is secured to the pulley 40 at 57 and to the pulley 49 of'condenser 23 at 58.

The form of condenser herein employed is one of the type which is intended to rotate through 180 and the points of attachment of the belts with their respective pulleys are therefore so locatedthat when the indicating arms point to zero the points of attachment lie in vertical lines passing through shafts of the condensers.

Thuswhen indicating arms 32 point to zero the point of attachment 55 is directly belowthe shaft of condenser 21 while the point of attachment 56 is directly above the shaft of the condenser 22. As the condenser shafts are rotated 180 to the right the point of attachment 55 moves to a position immediately above'the shaft in condenser 21 while the point of attachment 56 moves to a position immediately below the shaft of condenser 22. p The belts are thus connected with the pulleys in such a manner as to not interfere with the free rotational movement of the pulleys and attached condenser plates through the desired angle.

Each pulley ri'm'is provided, at the point of attachment of the belt, with a transverse slot 60 as best shown in Figs. 3 and 5. To attach the belt, the latter is doubled upon itself and the doubled end inserted through the slot after which a pin 61 is inserted through the loop formed by the doubled portion ofthe belt, andthe'belt is then drawn tight so as to bind the pin 61 in position. The slot 60 is preferably of such proportions as to hold the belt frictionally against sliding therein, so that even if the belt should break the pin 61 will be held against falling out of the loop. Obviously, if desired, the pin may be permanently secured to the pulley in any suitable maner. The belt is constructed of material which is flexible but which does not readily stretch and for this purpose a belt of woven fabric has been found togive good results. However, should any stretching of the belt occur, this may betaken up by means of a spring or other elastic means 62 which serves to hold the belt taut, so that upon rotation of the primary or master control dial 29 the several pulleyswill be caused to move in unison without any back-lash or lost motion. One

method of securing the spring 62 is illustrated in Fig. 6 wherein the ends of the belt 45 are folded over and secured by means of clips 63 to'form spring 62. p

From the foregoing it will be apparent that the present invention provides means by which the several movable elements of the tuning devices of a radio receiving apparatus may be moved or adj usted inunison whereby the periods of the several tuned circuits may be adjusted simultaneously and alike. l/Vhen the several indicating arms all point in the same direction the condensers are said to be coordinated, and it will be apparent that during movement of the condensers in imison the condensers will remain coordinated with one'another. Owing to slight mechanical or electrical imperfections in an apparatus, or variations in the construction thereof, it is sometimes desirable to move one of the tuning devices to a small extent, say one or two degrees, in order to obtain the best results or the clearest reception. The present invention also provides means for accomplishing loops 64 for receiving the such minor or secondary adjustmentsi By turning either of the readily accessible and readily operable secondary control dials 30 or 31 the condensers 22 and 23 maybe given slight adjustments independent of one another and independent of the condenser 21, and this without holding or otherwise locking either the master dial 29 or the other secondary control dial. WVhen thus adjusted the condensers 22 and 23 are no longer coordinated with the condenser 21 ina mechanical sense. When it is desired to tune the receiving apparatus to a different wave length or period the primary control dial 29 is operated to move the several condensers in unison, and here again it may be desirable to give the con-' densers 22 and 23 minor adjustments to obtain the clearest reception. However, since the extent of adjustment of the condensers 22 and 23 which may appear desirable may vary somewhat in amount or in direction at the different wave length settings of the primary control dial 29, it is generally preferable to return the several condensers into coordination with one another before they are moved from one position to another. Tofacilitate such coordination the condensers are provided with stop means or pins 65, 66, and 67, extending transversely to the plane of the condenser plates and lying against or ad acent the radial edge portion of the fixed condenser plates 24.

To coordinate the movable elements or plates of the condensers the main control dial 29 is turned to the left until its movable plate 25 strikes the stop 65. If the movable plate 25 of condenser 23 is in advance ofthe movable plate of condenser 21 then the former will strike the stop 67 and be held against further rotation to the left until the condenser V 21 strikes its stop 65. "Likewise if upon rotation to the right the condenser 22 is in ad vance of the condenser 21 then itsmovable plate 25 will strike the stop 66 and beheld against further rotation until the movable element 25 of condenser 21 strikes the stop 65. If both of the condensers22 and 23 are in advance of the condenser 21 in the same direction then the several condensers may be brought into coordination by simply'turning the main control dial 29 in one direction, but in any event the several condensers may always be brought into coordination by turning the main control dial 29 in both directions until it strikes the stop 65.

In Figs. 7, 8, 9 there is illustrated a'sec- 0nd embodiment of the inventionin which the condensers are actuated in unison by means of a suitable rack and pinion connection, the general arrangement and method of operation being similar'to that illustrated in Fig. 1. In this form of the invention a rack is supported upon [suitable guide blocks 71 secured to the panel 20 and is adapted to slide longitudinally upon-the supporting blocks 71. A driving pinion 72 is-pinned to the shaft 26 of'condenser 21 whereby upon rotation of the main control dial'29 the rack 7 O'will be given a longitudinal sliding movement in the guide blocks 71. The condensers 22 and 23- are connected in driving connection with the rack 7 0 through a'friction clutch arrangement, a' preferred construction being illustrated in Fig. 9. In this figure 73 is a pinion carried by'an insulating hub 74 loosely mounted upon the condenser shaft 27. A plate or washer 75 is 'pressedon the shaft 27 orpinned thereon as illustrated and a spring tension member 76 having spring arms is interposed between the hub 74: of the pinion and the disk 75 to establish a frictional connection between the shaft 27 and the pinion 73. The hub 74: and pinion 73 are held against longitudinal movement on the shaft 27 in a direction away from the disk 75 by means of a shoulder formed on the shaft which shouldermay be in the form of a C washer 76 secured to the shaft. i 1 i In this formof the invention the several condensers may be moved in unison by turning thejmain control dial29 and the" condensers 22 and 23 may be adjusted independently of the condenser 21 by turning their respective dials as will'be apparent from the'drawings.

According to the present invention any number of control devices, such as condensers, may be controlled from the single or master control dial 29. One arrangement is illustrated in Figs. 10 and 11 wherein 29-de'signates the main'control dialwhile 80 and 81 designate secondary control dials. In this form of the invention several secondary control devices may be connected withand operated from the main control dial 29 through the belts 44 and 45 mounted as already described in connection with Figsi-l and 2. Mounted upon theshaft of such secondary control de-; vice, as for example the device 80, is adouble pulley 82 which is mounted upon andhas a slidablefrictional connection with its shaft 83, the connection being illustrated in Fig;

11. The double pulley 82 is connected by means of a belt 84 with singlepulley 85 mountedupon-the shaft of control device 81 there being asuitable frictional connection between the pulley 85- and the shaft" of the control device 81 to permit independent movement of the latter. It is obvious that by mounting a double pulley upon the shaft of control dev1ce 81 such double pulley having a frictional connectionwith its shaft, the movement of the main control dial 29 maybe transmitted to still other control devices beyond the control device 81.i-

The number of control means and circuits that may thus be controlled from as'ingle dial is thus unlimitedandmaybe varied to any desired extent withinthe scopeof this in,-

vention merely-by the application, extension 0r r arrangement. of the-means ere dis: closed. A further such: application of the invention is illustrated in Figs. 12 to 15 in which Fig. 12 illustrates. by aschematic dia gram the circuits of a radio apparatus, which circuits may. be controlled in unisonor indea pendently in accordance; with the; presentinvention. Referring to this figure-reference characters 101, 102, and; 103 denotea series of resonant or tuned. circuits, these circuits being connectedin cascadethrough the medium, ofvacuum tubes. orelectron discharge devices 104 andtransformers-105 connected in a manner which is well understood by those skilled in the art. Thefirst resonant circuit 103 may be connected with an antenna circuit 106 through the medium of a ;tranS- former or coupler 107, whichmayor may not be variable. The circuit-s 101, 102, and 103 are provided with the variable inductances or variometers 1 08, 109, and 110 respectively, and are also,provided with the variablecondensers 21, 22, and 23 respectively. Each of the circuits 101, 102, and 103 may'thus be tuned by varying its inductance'or by varyor by varying both. of these factors-conjointl v 7 Figs. 13 to 15. illustrate means for controllingror varying the. variable inductances; and the variable condensers of, the, circuits 101, 102, and 103 in unison or for varying these devices or factors independentlyof one another. Referring first to Fig. 14 there is shown a primary or master control: dial 29 and secondary control dials 30 and 31 for con-v trolling the condensers 21, 22, and 23, the general construction and arrangement being similar to that illustrated in Fig. 1 except: in the particulars hereinafter pointed out. Fig. 13'illustrates means .forcontrolling. or varying the variable inductances or variometers v108, 109, and 110-in 111118011 or independently. The arrangement here is similar tothat-jillustrated. in Figs. 1 and 14, the variometer 108 carrying its capacity ingon its shaft a double pulley fixed thereon and the variometers 109; and 110.:carrying single pulleys 149which arein frictional dllV'? ing engagement withtheir'shafts; The puls leys- 140- and 149 areoperativelyconnected by the belts 144 and 145 in the manneripreviously described. Upon operation of" the dial 129 the several variometers 108,109,.and 1 10 aremoved in unison, and eachofthe variometersl09 and llOimay be varied independr, entlyby .operation of-its controldial 130 or 131 respectively, as will; bereadily :understood. a p

As best shown in Fig. 15, means is providedfor interconnecting the condenser -;controls and the variometer controls whereby all Pthese control or tuning meansmaybe operated in unison. For thispurpose the shaft26 ofthe master or primary-control29 is provided with asingle pulley 111- which is rigidly. secured thereon for rotation therewith, The shaft 126 of the Control 129,-qcarries asingle pulley.-

of the master or primary control dial 29.

Movement of the dial 29 is. transmitted to the condensers 22 and 23, thromgh the belts 44 and 45 respectively, andlikewise such movement is transmitted through the belt 113 to thedouble pulley 140 of the upper bank or unit ofcontrol devices, from which pulley 140 the; movement is transmitted th-roughthe belts 144 and 145tothevariometers 109 and 110 respectively. The frictional connection between the pulley 112- and the shaft 126 of thevariometer 108 is such that any movement of the master control dial 29;is conveyed to. the. shaft 126 and thus transmitted through. the double pulleyl40- to the several, variometers. Should itbe desired to-make independent adjustments of: the. condensers 22 and 23, this may be done. by operatingv thesecondary control'dials30;and 31, and likewise independent adjustments. of the variometers 109. and 110 may be effectedzbyoperation of the secondary control dials 130 and 131. In this connection, it will be. understood, of

course, thatithe variometers 108,1109, and 110 are provided with. suitable stop or coordinating means, such means corresponding in function and. general principle of operation with the coordinating means of. the .con-..

densers 21,22, and 23 and therefore not being illustrated. If desired, the: condensers. may be actuated .in unisonxandindependently of thevariometers by. moving the master control dial29 with one handw'hileholding the I control dial 129' against movement with the other hand, and in a similar manner the variometers may be moved in unison by actuating-ithe.-control'dial 129 while holding. the master control-.29 against movement. Such relative movements of the dials 29-and 129 is permitted through the. frictional connectionof the pulley 112. with its supporting shaft 126, as will be. readily understood. When the vari'ometers aremovedin unison byv oper ation of the control dial 129, this dial has the relation of a primary ormaster control with. reference to the secondary control dials 130 and 131 of the variometers 109. and 110. But with respect-to the master-control 'diall29 the control 129 may be considered as asecondarymay be used as the master dial for controlling all the tuning devices.

The form of the invention by which the capacity and inductance of the several reso- 5 nant circuits may be varied in unison or independently is particularly valuable Where it is desired to maintain the circuits at a high 'efiiciency for all frequencies within the range V of the apparatus. It is well understood that the resonance frequency of a circuit Varies directly as the product of the capacity and the inductance of the circuit and as a conse- ,quence the frequency may be varied by changing either the capacity or the inductance. However, the power factor of the circuit is at maximum or unity value only when the capacity and the inductance of the circuit are equal, that is, when the quotient of the capacity and. the inductance is unity. It is apparent, therefore, that the capacity and the inductance of a circuit should be changed simultaneously and equally to maintain the circuit at unity power factor, and further that any other method of adjustment of the circuit, as for example, the adjustment of the capacity alone while holding the inductance fixed, has the effect of decreasing the power factor of the circuit. According to the present invention the capacity and the inductance of theseveral circuits may be varied in unisonv so as to maintain the circuits at unity power factor, a featurewhich is of importance in securing the best results, particularly where the circuits are intended to receive signals havinga frequency of more than one million cycles per second, signals of this frequency being at the present time commonly termedlow wavelength signals.

While the forms of apparatus herein described constitute preferred embodiments of the invention, itisto beunderstood that the invention is not limited to these precise forms of apparatus, and that changes may be made in eitherwithout departing from the scope of the invention which is defined in the'appended-claims.

What is claimed is:

1. An apparatus of the character described comprising a pair of tuning devices each having a rotatably movable element, and control means for controlling the movements of said movable elements, said control means comprising a pair of'pulleys each mounted for rotation concentrically with respect to a movable element, a belt for connecting said pulleys in driving relation with one another, frictional driving connectionsbetween said pulleys and said movable elements whereby movement of 1 one of said pulleys is transmitted to both of said movable elements, one of said driving connections being so constructed and arranged as to permit the corresponding movable element to-be moved independently of the other movable element while the latter remains free. i

2. An apparatus of the character described comprising a pair of tuning devices each having a rotatable tuning element, and control means for controlling the movement's of said rotatable tuning elements, said control means comprising a pair .of pulleys mounted for rotation coaxially with respect to said elements, a belt fixedly secured to said pulleys for connecting said pulleys in driving relation with oneanother, driving connections between said pulleys and said movable elements whereby movement of one of said pulleys is transmitted to both of said movable elements, one of said driving connections being a constant frictionalv drive permitting the corresponding movable element to be moved independently of the other novable element While the latter remains ree.

3. An apparatus of the character described comprising a pair of tuning devices each having a movable element, and control means for moving said movableelements in unison, said control means comprising a pair of pulleys each connected with one of said movable elements, a beltriding on said pulleys andsecured thereto at diagonally opposite points, and means for maintaining said belt taut, the driving connection between one of said pulleys and its corresponding movable member being such as'tov permit independent movement of said movable member. r i

4:. An apparatus of. he character described comprising a pair of tuning devices each having a movable element, and control means for moving said movable elements in unison, said control means comprisinga pair of pulleys each connected with one of said movable elements, a belt riding on said pulleys to interconnect the same, and a yielding element connected in said beltto maintain the same taut, the driving connection between one of said pulleys and its corresponding movable member being such as to permit independent movement of said movable member. i.

5. An apparatus of the character described comprising a pair of control devices each having a movable element; primary means for moving said elements in unison,a sec ondary means for moving one of said elements independently ofthe other, and stop means for a rotatable element adapted upon movement of the. primary means to a predetermined position to coordinate said movable elements to a definite normal relationship with respect to one another.

6. An apparatus of the character described comprising three or 'more control devices each "having a movable element, a'single' primary means for moving "said elements in unison, said primary means having a fixed connection with one of said movable elements, secondary means for mov in the remaining movableelements indepen ently of the primary means, and atop means adapted upon n'ovement 'of' said primary means. to oneor bothof two predetermined positions tobring all said movable elements into'coordination with one another in a definite normal relationship with'respect to one another.

'7. Apparatus of the character described comprising a pair of tuning condensers each having a movable plate and a fixed plate, primary control means for movingsaid movable plates in unison, secondary control meansxfor moving one of said movable plates independently of the other movable plate, and stop means in the path of movement of said movable plates to bring the' same into coordination with one another in a definite normal relationship with respect to one another 2upon.the movement of said primary control means to a predetermined position.

8. Apparatus of the character described comprising a pair of tuning condensers each havingsubstantially sector-shaped 'movable and fixed plates, primary control means for moving said movable .plates in unison, secondary control means for moving one of'said movable plates independently of the other movable plate, and stop mea-ns in theipath of movement'o-f said movable plates to bring the same into coordination with one another radial edge ofone of said plates and in the path of'movement of the radial edge of the other'pl'ate; f i r '9. Anapparatusof the character described having a resonant circuit, adjustable-tuning means in said circuit comprising a plurality of -movableelement's, v primary" means for moving said movable elements in unison, sec- :ondary means for moving'one of said movable elements independently of the other movable element, and stop means for coordinating said movable-elements to a definite normal relationship with respect to' one another. I V

10. In an apparatus of the character de- 7 scribed, aplurality'of resonant circuits, a plurality oftuning means-in said circuits,primary means for adjusting said tuning means in unison,secondarymeans for adjustingfone of said tuning means independently of the other Etuhingmeans, and means for bringing thetuning means which-is controlled by said secondary means to a predetermined definite relationship with 'resp set to the other tuning means for coordinating the several tuning means; v

1'1-.'In anyapparatus of the character de scribed, I a plurality of reonant circuits, plurality ofituningmeansiin said circuits, .primary means for-adjusting saidjtuningzmeans in -uni'son, secondary means for adjusting one of said'tuning means independentlyof the other tuning means, and mea-ns operable merely by the operation of said-primary means to automatically coordinate the several tuning means. 7

12.111 a'radio receiving apparatus, a series ofr-tumng condensers having movable plates rotatable about spaced parallel axes,

movable plates of each condenser coaxially therewith, and 'a series of belts connecting each pulley to an adj acentpulley toprovide for equal and similar'movement of all said condensers when said one member is moved,

and a spring connected in each'belt to main-" 'tain the same taut.

13. A radio tuning device comprising a pair of tuning devices each having amovable element, and control means for moving said movable elements unison, said control means comprising a pair ofipulleydrum's each connected with oneof saidmovable elements, a belt riding on said pulleys and secured thereto at diagonally opposite points so as to permit 180 rotation ofsaid pulley drums in a definite predetermined relationship. l

14. A radio tuning device comprising a pair of tuning devices each having 'a movable element, and-control ineansior moving said 'meanscomprising apair of pulley drums eachconnected with one of said'movableelements, a beltriding; onsaid pulleys andsecured thereto at diagonally opposite points so as topermit 180 rotation of saidpulley drums in a definite predetermined relationship, and a springrinterposed infsaid belt for maintaining the same taut.

15. A radio tuning device comprisinga pair of tuningdevices each'havingamovable element, and control means "for'moving said movable elements in .unison, said control means comprising *apair of pulley drums each connectedwith one-of said movable elements, said drums being each provided with a transverse slot, a belt ridingon-said drums and having portionsh'eld within said slots so that said pulley drums are secured at diagonally opposite points to 'the'belt to Epermit 180 rotation of said pulley 'drums i'n-a definite predetermined relationship.

16. In a radio receiving apparatus, aplurality of adjustable tuning circuits -each circuit having one or :more tuning devices and at least one of said circuits having acondenser tunin'gxdevice and an impedance tuning device, a single means for adjusting, the several tuning'devices ofsaid' last mentioned circuit and of said-circuits in :unison, and means for adjusting" the severaldevi'ces of said last mentioned circuit and of s'aidcirn apparatus of" the charactermde s30 movable "elements in junison, said vcontrol each having a rotatable tuning element and control means for controlling the movements of said rotatable tuning elements, said control means comprising a pair of pulleys mounted for rotation coaxially with respect to said elements, a belt interconnecting said pulleys in driving relation with one another, driving connections between said pulleys and said movable elements whereby movement of one of said pulleys is transmitted to both of said movable elements, one of said driving connections being a constant frictional drive permitting the corresponding movable ele ment to be moved independently of the other movable element while the latter remains free and a control knob connected to said corresponding movable element.

In testimony whereof I hereto afiix my signature.

ORIN E. MARVEL. 

